In an internal combustion engine equipped with an exhaust purifying system using a catalyst, harmful substances in an exhaust gas are generally purified by the catalyst in a highly efficient manner. Therefore, it is fundamental to control a mixing ratio of air and fuel in a mixture to be burned in the internal combustion engine, that is, an air-fuel ratio. For controlling such an air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage in the internal combustion engine, and feedback control is performed in such a manner as to make the air-fuel ratio detected by the air-fuel ratio sensor be equal to a predetermined target air-fuel ratio.
On the other hand, there are some cases where, since air-fuel ratio control is usually performed applying the same control amount to each of all the cylinders in a multi-cylinder internal combustion engine, an actual air-fuel ratio varies between cylinders even if the air-fuel ratio control is performed. When a degree of the imbalance is small at this time, since the imbalance can be absorbed by air-fuel ratio feedback control and the harmful substances in the exhaust gas can be purified also in the catalyst, the imbalance has no adverse influence on exhaust emissions and raises no particular problem.
However, when the air-fuel ratio varies largely between the cylinders due to a failure of a fuel injection system in a part of the cylinders or the like, the exhaust emission is deteriorated, thus raising the problem. It is desirable to detect the imbalance in the air-fuel ratio as large as to thus deteriorate the exhaust emission, as abnormality. Particularly in a case of an internal combustion engine for an automobile, for beforehand preventing a travel of a vehicle in which the exhaust emission has deteriorated, it is requested to detect the imbalance abnormality in the air-fuel ratio between the cylinders on board (so-called OBD; On-Board Diagnostics), and there is recently the movement of legalizing such detection of the imbalance abnormality on board.
For example, in an apparatus described in PTL 1, when it is determined that abnormality in an air-fuel ratio occurs in any of cylinders, injection time of fuel injected to each cylinder is shortened for each predetermined time until the cylinder in which the abnormality in the air-fuel ratio has occurred misfires, thus specifying an abnormal cylinder.
Incidentally in a case where the abnormality in the air-fuel ratio occurs in any of the cylinders, when a fuel injection quantity is forcibly increased or decreased in the corresponding cylinder, a rotation variation in the corresponding cylinder becomes remarkably large. Therefore, by detecting an increase in such a rotation variation, it is possible to detect the imbalance abnormality in the air-fuel ratio.
However, the increase or decrease in the fuel injection quantity results in deterioration of an exhaust emission more than a little. Therefore, it is desirable to perform the increase or decrease in the fuel injection quantity at timing for not deteriorating the exhaust emission as much as possible.
Therefore, the present invention is made in view of the foregoing problem and an object of the present invention is to provide an apparatus for detecting imbalance abnormality in an air-fuel ratio between cylinders in a multi-cylinder internal combustion engine which can prevent exhaust emission deterioration due to execution of abnormality detection as much as possible.